Radiotherapy has been used to treat cancer in the human body since early 1900. Even though radiation of cancer tumours is known to be efficient, mortality rate for many cancers remained virtually unchanged for a long time. The major reasons for this have been the inability to control the primary tumour or the occurrence of metastases. Only by improving the local control may the treatment be more effective. In the last years Treatment Planning Systems, TPS, in Radiation Therapy have developed extensively and is now able to take into account the anatomy of the specific patient and in a time efficient way plan a more optimised treatment for each individual patient, homogenous dose to the target and minimum dose to risk-organs.
The treatment technique to deliver this optimised treatment is more complicated than conventional treatments because each field must be modulated laterally in intensity and thereby compensate for the heterogeneity and contour of the patient, the technique is called IMRT—Intensity Modulated Radiation Therapy. The delivery can be done using compensators, filters that reduce the intensity to a predefined level in each part of the field due to attenuation of the primary photon beam. However when using several fields (4-8), each field requiring individual compensators, this technique is time consuming and requires a lot of effort. Additionally the attenuation of the beam also causes unwanted change of the spectral distribution in the beam, thereby complicating the whole process. The most common way to deliver the IMRT fields will therefore be to use the MLC (Multi Leaf Collimator) a device that consists of thin blocks (Leafs) that can be individually positioned to block a small part of the field and thereby shape the beam in the lateral direction to various irregular shapes. By moving the Leafs during the treatment each part of the treated volume will be irradiated during various time and thereby the intensity over the treated area is modulated.
The new treatment technique however impose that the patient is exactly in the position expected, something not always easy to achieve. Additionally the requirements on accurate dose delivery increase and thereby the requirements on quality control (QC) of the treatment machine, the planning process and finally during the treatment, increase. New verification and QC are to be used. However very little has been published on measurements during treatment, In Vivo dosimetry.
Traditional In Vivo dosimetry, measuring with a detector on the skin of the patient to predict the dose inside the patient is very demanding already with a fixed field (conventional therapy) due to limitations in the TPS (Treatment Planning System) to predict the dose distribution in the region of the patient where externally generated secondary electrons contribute significantly to the delivered dose e.g. build-up region (the part where the beam enters the patient and to a depth 5-35 mm into the patient). Thereby neither the surface or skin dose or the dose in air up-streams the patient can be accurately predicted by the TPS in fixed fields and the difficulty increases with a dynamically delivered treatment. In fixed fields this is solved either by using a special design of the detector, by general calibration or a combination of the two. In IMRT treatments it is not that easy to handle this either by general calibration or design due to the fact that the varying intensity in the field is patient specific. The traditional In Vivo dosimetry is normally not used at each fraction and thereby the perturbation of the specially designed detectors becomes negligible. The small margins in IMRT treatments require extended dosimetry and quality control also at each fraction to minimise the uncertainties and therefore the perturbation of the detectors used in conventional therapy becomes significant. Additionally when using IMRT, measurements must be done in many points to verify the field's topography and the lateral position of the detectors is very critical. To simplify the problem it has been suggested to just measure the fluence in air. However, then the discrepancy from the predicted values will be difficult to judge due to lack of understandable quantification.
Alternatively to traditional in vivo dosimetry it has been proposed to use imaging systems positioned down-streams the patient, film or EPID (Electronic Portal Imaging Device) where the device is calibrated to measure dose. Such a method is discussed in “Portal dose image prediction for dosimetric treatment verification in radiotherapy I: and algorithm for open beam”, by K. I. Pasma et al., Medical Physics 25(6), pages 830-840, 1998. A comparison can then be done with calculated dose distribution using e.g. the TPS (Treatment Planning System) at the position of the measuring device. An example of this is described in “In Vivo dosimetry for prostate cancer patients using an electronic portal imaging device; demonstration of internal organ motion”, by M. Kroonwijk et al., Radiotherapy and Oncology. 49(2), pages 125-132, 1998. Another alternative is to calculate the dose distribution in the patient from the measured dose distribution in the EPID. This is disclosed in “Modelling the dose distribution to an EPID with collapsed cone kernel superposition”, C. Vallhagen Dahlgren et al., Workshop in Uppsala, Mar. 13, 2001, organised by the company MDS Nordion.
The latter has the benefit of providing data that is more easily understandable. However measurement down-streams the patient alone will always be less accurate than combined with measurements up-streams the patient and will thereby not distinguish if the deviation was caused due to incorrect dose delivery by the treatment machine or due to positioning errors or change in anatomy of the patient (the patient might loose weight etc. from original diagnostics). The latter is important not least in order to analyse the root of the deviation and thereby to prevent it from occurring in the next treatment-fraction (normally a patient receives 30 fractions before the treatment is completed).